1. Technical Field
This disclosure relates to wireless communication systems that transmit and receive at the same time, as well as to leakage of the transmitted signal into the received signal in such systems.
2. Description of Related Art
In wireless communication systems, a wireless receiver (RX) may receive a signal while a wireless transmitter (TX)—which may be collocated, co-site, or in close vicinity—may simultaneously transmit a different signal. For example, in a frequency division duplex (FDD) radio communication system, the TX and the RX of the same radio may operate simultaneously in two different frequency bands, ftx for TX and frx for RX, respectively. Another example is a smart phone that has multiple radios, such as a Wi-Fi radio and cellular radio, that may operate simultaneously. This may result in the RX of one radio receiving a signal in one frequency band frx, while the TX of the other radio is transmitting a signal in another frequency band fINT.
In such scenarios, an important performance metric for the radio communication system may be the “isolation” between the TX and the RX that are operating simultaneously. The isolation can be particularly important within the TX frequency band and within the RX frequency band. Any leakage that reaches RX within the TX frequency band or the RX frequency band may interfere significantly with the receiver. The minimum required isolation may depend on the application and the scenario. For example, in a commercial FDD radio, TX to RX isolation of 50 dB or more may be required in the TX and the RX frequency bands. Without adequate isolation, the aggressor TX signal may significantly deteriorate the sensitivity of the victim RX and ultimately prevent its proper operation.
In FDD radios, a popular approach to providing TX-RX isolation is to use a frequency duplexer. The duplexer is a three port electrical network. One port is typically connected to the antenna (ANT), one port is connected to the TX output, and one port is connected to the RX input. A common challenge in duplexer design is to achieve low insertion loss from TX to ANT and from ANT to RX, while providing high isolation from TX to RX. However, meeting this requirement may require costly resonator technologies, such as Bulk Acoustic Wave (BAW) resonators. In addition, with emerging growth in the number of frequency bands that an FDD radio may need to support, the number of duplexers in a radio may grow proportionally, which may result in a complex, non-scalable radio system. Consequently, a tunable duplexer that can be electronically tuned to address more than one band can be extremely desirable to reduce the complexity and cost of the radios. However, acoustic resonators can be difficult to tune, while other types of tunable resonators may not provide sufficient isolation without significant losses and/or without undue bulk.
Isolation of aggressor TX and victim RX has also used a combination of separation in frequency domain and customized filtering. The isolation is typically tied to a specific platform and frequency band of the radio it supports, which can be limiting as the radios become more complex and support more bands.
Another approach to enhancing isolation between TX and RX is to generate a cancellation signal that fully or partially matches the amplitude of the leakage signal from the aggressor TX to the victim RX, but with the opposite phase. The cancellation signal is then combined with the leakage signal to cancel it out and enhance isolation.
This cancellation method may require an active circuit, such as an amplifier, in the path that creates the cancellation signal to match the leakage signal. Introduction of an active circuit can be extremely undesirable because it can introduce excess noise, limit power handling due to nonlinearity, and/or consumes power. Furthermore, such an approach may cancel the leakage signal downstream of the signal path in the victim RX. This may require the up-stream signal path of the victim RX to be linear which, in turn, may impose a limit on the RF power handling capability of the system. If the combination with leakage signal is achieved in a feedback cancellation, moreover, a significant stability requirement may also need to be accounted for.
There is a class of radio communication systems, where the TX and RX operate simultaneously at the same band, also sometimes referred to as the same channel. In such scenarios, there may not be any frequency separation between RX and TX. Therefore, TX-RX isolation may not be achieved with any bandpass filtering or similar circuits. Proper operation of RX may then largely depend on the cancellation of the self-interfering TX signal of the same radio. Depending on the required sensitivity for the RX and the output signal power of the TX, total isolation of 100 dB to 130 dB may be required. Such high level of isolation may be difficult to achieve with one cancellation step at the radio front-end. Furthermore, additional filtering may be necessary to filter other interfering signals arriving at the RX in other frequency bands.